Sanitary supplies such as paper diapers and sanitary napkins, and absorbent articles such as so-called pet sheets for pet animals are generally produced using a lot of synthetic resins and modifying agents, such as top sheets, back sheets, hot-melt adhesives, stretchable materials, water-absorbent resins and pulp fibers, and therefore can sometimes generate odor originating from raw material components. Since sanitary supplies are applied to the human body, even slight odor may make users uncomfortable and it is required to make the sanitary supplies odorless. In particular, a water-absorbent resin as a main material of sanitary supplies has slight odor originating from a substance which is used in the production process and the odor is likely to be generated at the time of water absorption. Therefore, it is strongly desired to reduce odor of a water-absorbent resin per se.
There are known, as a water-absorbent resin to be used in sanitary supplies, a partially neutralized polyacrylic acid, a neutralized product of a starch-acrylic acid graft polymer, a hydrolyzate of a starch-acrylonitrile graft polymer, a saponified product of a vinyl acetate-acrylic acid ester copolymer and the like. It is considered that the cause of odor generated by the water-absorbent resin lies mainly in the production method. That is, since the water-absorbent resin to be used for sanitary supplies is generally in the form of a powder, it is often produced by a reversed-phase suspension polymerization method by which a powdered water-absorbent resin is easily produced. According to the polymerization method, an aqueous monomer solution is suspended in a dispersion medium and the obtained suspension is polymerized, and thus the dispersion medium can be one reason of odor from the water-absorbent resin.
There are known, as the method for producing a water-absorbent resin by a reversed-phase suspension polymerization method, various methods, for example, a method in which a sucrose fatty acid ester is used as a protective colloidal agent when an α,β-unsaturated carboxylic acid and an aqueous solution of an alkali metal salt thereof are polymerized in a petroleum-type hydrocarbon solvent in the presence or absence of a crosslinking agent using a radical polymerization initiator (Patent Literature 1), a method in which a polyglyceryl fatty acid ester having an HLB of 2 to 16 is used as a surfactant when an α,β-unsaturated carboxylic acid and an aqueous solution of 25% by mass or more of an alkali metal salt thereof are polymerized in a petroleum-type hydrocarbon solvent in the presence or absence of a crosslinking agent using a radical polymerization initiator (Patent Literature 2) and the like. However, all the production methods aim at improving water absorption capacity and do not pay attention to odor, and it cannot be said that any obtained water-absorbent resin suppresses odor.
Also, sanitary supplies have a problem of odor from another point of view. That is, since sanitary supplies aim at absorption of body fluids such as human urine and blood, there is a problem that suppression of odor, for example ammonia odor, generated with the lapse of time after absorption of a body fluid is required. It is also considered that odor is suppressed by using a sterilizer. However, in sanitary supplies to which harmlessness is particularly required, odor is generally suppressed by controlling the pH of a water-absorbent resin per se, or the pH of an absorbent material using a water-absorbent resin. For example, Patent Literature 3 describes a method in which a pH adjustor such as an organic acid like citric acid or adipic acid, a polyacrylic acid having a neutralization degree of 50% or less, or an ion-exchanging cellulose layer is added in an absorbent material. Also, Patent Literature 4 describes a method in which a water-absorbent polymer having a neutralization degree of 70 mol % or less is coated by being brought into contact with an acidic component of an aqueous organic acid solution such as an aqueous citric acid solution. Furthermore, Patent Literature 5 describes a method in which fine particles are fixed on a surface of an acid group-containing water-absorbent resin particle by treating the resin particle with an acid group-containing radical polymerizable compound thereby adjusting an acid group neutralization index at the surface of an absorbent to less than that in the interior thereof.
However, the method of Patent Literature 3 has a problem in economical efficiency since the pH adjustor is expensive. Moreover, since the pH adjustor per se has no liquid absorbency, absorption characteristics of an absorbent material may be impaired. The method of Patent Literature 4 has a problem in economical efficiency since an acidic component is expensive, and also the coating step is complicated. Furthermore, the method of Patent Literature 5 has low practicability since the production process is complicated and troublesome, and also the unreacted acid group-containing radical polymerizable compound may remain and therefore application of the obtained absorbent to sanitary supplies has safety concerns.